This invention relates to an improved Raman oscillator capable of producing stimulated Raman scattering (SRS) at high pulse repetition frequencies.
Light scattering spectroscopy of various substances is well known and involves the scattering of a light beam which traverses a transparent dielectric sample (solid, liquid or gas), the scattering being caused by the different dielectric characteristics of the sample. This linear or thermal scattering of light has been observed without lasers for a long time. With the advent of the laser and its high intensity, collimation, and monochromaticity, research in light scattering spectroscopy has been greatly facilitated.
SRS concerns scattering from the optic vibrational modes and is achieved by directing a pump beam from a laser, usually pulsed, through a Raman medium such as crystal. The pump beam decomposes into a Stokes wavelength output, which is photon-like, and a Raman wavelength output. The Raman output is closely tied to a natural vibrational mode of the crystal and can be either purely phonon-like, or partially phonon-like and photon-like in character.
In the decomposition process, conservation of energy must be satisfied as stated in the frequency equation EQU f.sub.P = f.sub.S + f.sub.R ( 1)
where f.sub.P, f.sub.S and f.sub.R are the pump, Stokes and Raman frequencies, respectively, and conservation of momentum must be satisfied as stated in the wavevector equation EQU k.sub.P = k.sub.S + k.sub.R ( 2)
where k is the wavevector for each of these waves. Since SRS results in the generation of both Stokes and Raman waves, the resonator or oscillator which produces these waves is known as either a Raman or a Stokes resonator or oscillator.
Stokes outputs represent coherent optical outputs at new wavelengths that may not be available directly from existing lasers. The Stokes output can have several different components at successively longer wavelengths known as the first Stokes, second Stokes, third Stokes, etc. The first Stokes output is generated as per Equations (1) and (2) above. In certain circumstances, this newly generated first Stokes beam can itself act as a pump for SRS thereby producing a so-called second Stokes output. If the second Stokes beam in turn acts as the pump for still further SRS, a third Stokes beam is generated, and so forth.
A Raman oscillator configuration that has been successfully tested is described in an article by E. O. Ammann and J. Falk in Applied Physics Letters, Vol. 27, No. 12, Dec. 15, 1975, pages 662-664, entitled "Stimulated Raman Scattering at kHz Pulse Repetition Rates" and comprises a separate Raman resonator within the laser cavity. This invention is directed toward an improvement to that Raman oscillator.